Tantalum base alloys

ABSTRACT

This invention is directed to tantalum-base alloys having superior elevated temperature strength and high resistance to creep deformation at elevated temperature in a high vacuum or space environment. The alloys comprise from 4 to 8.5 percent tungsten, up to 3 percent of at least one of the group consisting of molybdenum and rhenium, up to 1.5 percent of at least one of the group consisting of zirconium and hafnium, up to 0.04 percent of at least one of the group consisting of carbon and nitrogen and the balance essentially tantalum.

United States Patent [191 Buckman, Jr.

[111 3,791,821 [451 Feb. 12, 1974 I TANTALUM BASE ALLOYS [75] Inventor: Raymond W. Buckman, Jr.,

Pittsburgh, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Oct. 30, 1968 21 Appl. No.: 771,714

Related US. Application Data [63] Continuation-in-part of Ser. No. 598,941, Dec. 5, I966, abandoned, which is a continuation-in-part of Ser, No. 454,650, May I0, 1965, abandoned.

[52] US. Cl. 75/174, 148/32 [51] Int. Cl C22c 27/00, C22f l/l8 [58] Field of Search 75/174; 148/32, 32.5, 133

[56] I References Cited UNITED STATES PATENTS 4/1968 Chang et al 75/l74 7/l968 Ammon et al. 75/174 OTHER PUBLICATIONS Ammon et al., Pilot Production and Evaluation of Tantalum Alloy Sheet," WANLPRM004, June 15, I963, rclied on pages 50, 51, 53,55, 58, 62, 78, 79 & 80.

ASD-TDR62594, Investigation of Tantalum and Its Alloys, May 1963, relied on pages 93, 95, 96, 101, I03, I04, I07 &108.

Primary Examiner--Charles N. Lovell Attorney, Agent, or FirmF. Shapoe; R. T. Randig [57] ABSTRACT This invention is directed to tantalum-base alloys hav ing superior elevated temperature strength and high resistance to creep deformation at elevated temperature in a high vacuum or space environment. .The 211' loys comprise from 4 to 8.5 percent tungsten, up to 3 percent of at least one of the group consisting of molybdenum and rhenium, up to 1.5 percent of at least one of the group consisting of zirconium and hafnium, up to 0.04 percent of at least one of the group consisting of carbon and nitrogen and the balance essentially tantalum.

9 Claims, 3 Drawing Figures SOLID SOLUTION TOZC SOLID SOLUTION+T0 C+HfC JSOLI D SOLUTION +HfC ATOMIC AND WEIGHT PERCENT HAFNIUM PAIENIEBFEBI 21924 SOLID SOLUTION +TG2C SOLID SOLUTION +T0 C+HfC SOLID SOLUTION +HfC FIG.I.

-.o| c v ATOMIC AND WEIGHT PERCENT HAFNIUM TESTED AT.

TESTED ATI 2400F I5,000 psi mo TORR HAFNIUMI+ ZIRCONIUM) ATOM PERCENT RHEN'UM CONTENT'IWEIGHT PERCENT) IN TANTALUM FIG.3.

FIG.2.

INVENTOR WITNESSES M Y m L o 7%w WV mYwfl "w M, R

TANTALUM BASE ALLOYS This application is a continuation-in-part of application Ser. No. 598,941, filed Dec. 5, 1966, now abandoned, which in turn was a continuation-in-part of application Ser. No. 454,650, filed May 10, 1965,.now abandoned, assigned to the same assignee as the present invention. I

The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435;- 42 U.S.C. 2457).

The development of reliable space power generation systems requires the availability of alloys which can withstand the rigorous environment in which they are to be used, namely high operating temperatures, nearvacuum pressures, high operating stresses and the corrosive effect of liquid alkali metals. The alloys employed for the tubing and sheet for such space power systems are required to have a combination of characteristics including good fabricability, good weldability, strength at elevated temperatures, good resistance to creep deformation at elevated temperatures in high vacuum and corrosion resistance to liquid alkali metals.

, Tantalum-base alloys containing reactive metal additions such as zirconium and hafnium have been and are presently under development in the industry and such alloys exhibit good fabricability and corrosion resistance to liquid alkali metals. These alloys were designed primarily for short-time applications; that is, for periods of use amounting to about ten hours, where solid solution alloy additions are extremely beneficial in that they confer high tensile strength upon the alloys. High tensile strength, however, is not necessarily indicative of good resistance to creep deformation. This is particularly true where the operating environment in which the alloy is expected to function is a high vacuum, for it is known that the environment pressure has a significant effect on creep properties, and, in some cases, high vacuum reduces resistance to creep deformation.

Accordingly, it is an object of this invention to produce fabricable tantalum-base alloys which are resistant to the corrosive effect of liquid alkali metals and have good strength at elevated temperatures and high resistance to creep deformation at elevated temperature in a vacuum environment.

A more specific object of the present invention is to provide a tantalum-base alloy having dispensed secondary phases comprising a major proportion of dimetal carbides and reactive metal nitrides and which is characterized by improved creep resistance, good fabricability and weldability and a controlled ductile-brittle transus temperature below room temperature.

Other objects of this invention will become apparent to those skilled in the art when taken in conjunction with the following description and the drawings in which:

FIG. I is a portion of the pseudo-ternary phase diagram of the tantalum rich corner of the (Ta W Re)-Hl-C alloys at the 1315C (2900F) isotherm;

FIG. 2 is a graph illustrating the time to 1 percent total strain versus the hafnium plus zirconium content and carbide character under a stress of 15K psi and in invention, as circumscribed by the appended claims,

have a close packed hexagonal crystallographic configuration with lattice constants substantially within the range between about, a,,, 3.10 to about 3.14 A. and, c about 4.92 to about 4.96 A. These lattice parameters confirm that there is only minor substitution of hafnium and/or tungsten for tantalum in the formation of the dimetal carbide having the general formula (Ta, Hf, Mo, Re, Zr, W)2 C where x is substantially less than 1 and approaches zero flhen the composition iswmaintain ed within the limits set forth hereinafter the preferred dimetal carbide is formed as will be evident from inspection of FIG. 1.

The alloys of the invention include, in addition to tantalum, the element tungsten and some or all of the elements molybdenum, rhenium, hafnium, zirconium, carbon, and nitrogen.

Broadly stated, these alloys comprise, by weight, from 4 percent to 8.5 percent tungsten, at least one element selected from the group consisting of molybdenum and rhenium, molybdenum when present amounting to up to 1.25 percent, rhenium when present amounting to up to 3 percent, the total amount of molybdenum plus rhenium being at least 0.5 percent, but not exceeding 3 percent, up to 0.75 percent zirconium, up to 1.5 percent hafnium, the total of zirconium and hafnium not exceeding 1.5 percent, trace amounts to about 0.03 percent nitrogen, trace amounts to about 0.03 percent carbon, the total carbon and nitrogen being at least about 0.01 percent but not exceeding about 0.04 percent, and the balance essentially tantalum, except for incidental impurities.

Each of the alloying components performs a specific function within the alloy of the present invention. Thus tungsten within the stated range improves the strength of the solid solution. It is desired to maintain at least 4 percent for effective solid solution strengthening. Where the tungsten content is increased to beyond about 8.5 percent the fabricability properties and especially weldability appear to deteriorate. Up to about 1.25 percent molybdenum and up to about 3.0 percent rhenium may also be present to aid in the solid solution strengthening; however, such additions must be care-' fully balanced in order to obtain an outstanding combination of properties. This is clearly demonstrated by reference to FIG. 3.

It is noted in FIG. 3 that where the rhenium content is increased up to about 0.5 percent the creep properties are vastly improved. Further increases in the rhenium content provide only a smaller improvement in the creep properties up to about 3.0 percent rhenium. However while the creep properties are improved the as-welded ductility as exemplified by the ductile-brittle transition temperature (DBTT) is raised from about 225F at the 1 percent rhenium level to about room temperature for rhenium contents of about 3 percent.

Consequently, the preferred rhenium content lies within the range between about 0.5 percent and about 1.5 percent although useful alloys can be produced over the entire range depending upon the ultimate requirements in use.

The alloy of the present invention also contemplates the presence of reactive metals hafnium and zirconium with the hafnium being the preferred component for obtaining optimum fabricability and weldability. No substantial difference appears to exist between hafnium and zirconium insofar as the creep properties are concemed. Up to 0.75 percent zirconium may be present and up to 1.5 percent hafnium can be present with the proviso that the sum of the zirconium plus hafnium does not exceed 1.5 percent. The reason for these limits appear hereinafter.

Hafnium and zirconium, while entering into solid solution can also react with the carbon present to form a series of carbides depending upon the amount of reactive metal and carbon which is present. The higher amounts of hafnium and/or zirconium necessary to stabilize the monometal carbide is effective for exerting a powerful influence upon the creep properties of the alloy of this invention.

Referring now toFIG. 2, the effect of the hafnium and zirconium on the creep properties is illustrated. Thus where the hafnium and zirconium are maintained within the limits set forth it may be observed the alloy will possess good creep properties. Where however, the upper limit is exceeded a marked deterioration in the creep properties is evident. The limits of the phase boundries set forth in FIG. 1 as respects the crystallographic character of the carbides present are also shown in FIG. 2. Thus it will become apparent that in order to obtain outstanding creep properties only the dimetal carbide should be present. Accordingly, the limits set forth hereinbefore for the hafnium and zirconium have been selected so that only the dimetal carbide is present in the alloy of the present invention.

In addition to limiting the solution strenghening components to the limits set forth hereinbefore, it is also preferred to limit the total solute components to a maximum of percent, that is, the sum of the tungsten, molybdenum, rhenium, hafnium and zirconium must be limited to 10 percent with the individual components also being limited to each of their respective ranges.

The alloy of the present invention envisages the presence of nitrogen in amounts of up to 0.03 percent and carbon in amounts of up to 0.03 percent with the total carbon and nitrogen limited to 0.04 percent maximum. Both carbon and nitrogen react to form carbides, nitrides and carbonitrides. Of primary importance, however, the phases so formed must exhibit a major proportion of the dimetal carbide in order to obtain optimum creep properties. By inspection of FIG. 1 it will be observed that only the dimetal carbide phase is present when the alloy composition is balance as described hereinbefore.

Reference is again directed to FIG. 3 which also demonstrates the effect of carbon and nitrogen on the creep properties. Thus in FIG. 3 the curve 10 illustrates the level of properties where the carbon plus nitrogen content is less than 50 ppm whereas'curve 12 illustrated the level of properties where the carbon plus nitrogen I is within the range between 200 and 300 ppm. FIG. 3

clearly demonstrates that where the dispersed secondary dimetal carbide phase is present, outstanding creep results areobtained by controlling the alloying components as set forth hereinbefore.

One particularly useful group of alloys of this invention contains the above specified elements in the ranges of, by weight, from 4 percent to 8 percent tungsten, from 0.5 percent to 1.25 percent molybdenum, from 0.5 percent to 2.5 percent rhenium, from 0.1 percent to 1.5 percent hafnium, from trace amounts to 0.75 percent zirconium, the total of zirconium and hafnium not exceeding 1.5 percent, trace amounts to 0.03 percent carbon, from 0.01 percent to 0.03 percent nitrogen, the total carbon and nitrogen being at least 0.01 percent but not exceeding 0.04 percent, and the balance essentially tantalum with small amounts of incidental impurities. Another preferred composition within these ranges comprises, in weight percent, about 5.5 percent to 5.8 percent tungsten, from about 0.6 percent to 0.9 percent molybdenum, about 1.4 percent to 1 .6 percent rhenium, about 0.2 percent to 0.3 percent hafnium, about 0.1 percent to 0.2 percent zirconium, about 0.02 percent carbon, about 0.02 percent nitrogen and the balance essentially tantalum with small amounts of incidental impurities. Expressing the composition of this preferred alloy in atomic percent the tungsten amounts to about 5.6 at the molybdenum about 1.4 at the rhenium about 1.5 at the hafnium, zirconium, carbon and nitrogen are present in equal amounts, that is about 0.25 at Still another preferred composition comprises, in weight percent, from about 7 to 8.5 percent tungsten, from 0.5 to 1.5 percent rhenium, from 0.5 to 1.25 percent hafnium, from 0.02 to 0.03 percent carbon and the balance essentially tantalum with small amounts of incidental improvements.

A specific outstanding alloy composition comprises, by weight, about 8 percent tungsten, about 1 percent rhenium, about 0.7 percent hafnium, about 0.025 carbon and the balance essentially tantalum. In this alloy nitrogen is substantially absent. The creep rate properties, namely time to 1 percent strain at elevated temperatures under high loads in a high vacuum, have proven to be outstanding. Because of fabricability properties superior to any of the other alloys of this invention, this composition is particularly advantageous.

Other desirable alloy compositions within the scope of the invention comprise, by weight, about 6.3 percent to 7.2 percent tungsten, one element selected from the group consisting to about 0.7 percent to 1 percent, rhenium, when present amounting to about 1.4 percent to 1.6 percent, about 0.2 percent to 0.3 percent zirconium, up to about 0.4 percent to 0.6 percent hafnium, about 0.02 percent nitrogen, up to about 0.02 percent carbon and the balance tantalum except for incidental impurities.

Alloys of the invention were prepared having the compositions set forth in Table I. Table I also includes the composition of the highly meritorious tantalumbase alloy known in the art as T-222 alloy for comparative purposes. The quantities of each element are ex- TABLE 1 Alloy W g Mo Re Zr Hf C N Ta A 5.7 0.7 1.56 0.15 .25 .017 .02 Bal. B 7.1 1.56 0.26 .02 B31. c 7 0.85 0.26 .02 Bal. D 6.5 0.8 0 26 0.5 .017 02 Bal. E 8 1 0.7 .025 Bil. F 9 1 .025 Bal. T-222 10 2.5 .01 B21.

residual carbon level 2-30 ppm.

Each of the exemplary alloys A, B, C, D and E of From the above short time data it is seen that alloys Table I has substantial merit in its own right. Each of A, B, C and D of this invention are equal or somewhat these alloys is representative of a different type of alloy superior to the alloy T-222 in strength at the 2400F within the broad invention. Compositions of the Alloy temperature level while retaining ductility to an accept- A and Alloy E type have been more generally defined able degree. Alloy E is somewaht lower in strength than above as preferred compositions. Set forth below are the T-222 alloy, but equal to or better than T-222 alloy definitions of each of the more limited ranges, within in ductility. the broad range, of the alloy types represented by Al- The materials tested above were obtained from nonloys B, C and D. consumable arc melted material. However, material of Thus, desirable alloys of the Alloy B type have the similar quality can also be produced by consumable arc compositions of, by weight, about 6.8 percent to 7.2 melting or electron beam melting techniques as well. In percent tungsten, about 1.4 to 1.6 percent rhenium, any case, the raw materials used are of high purity; for about 0.2 to 0.3 percent zirconium, about 0.02 percent example, the tantalum employed 99.9 percent pure nitrogen, and the balance tantalum. with no more than 20 ppm each of the relatively vola- Compositions of the Alloy C type have substituted tile impurities Fe, Si, Cr and Pb. The tungsten and motherein about 0.7 to 1.0 percent molybdenum for all of lybdenum used were also over 99.9 percent pure. The the rhenium in the Alloy B type, but are otherwise of hafnium used contained as much as 2.5 percent by weisimilar composition. I ght of zirconium. The as-melted alloys contain 20 to Compositions of the Alloy D type contain about 6.3 100 ppm each of carbon and nitrogen without intento 7.2 percent tungsten, about 0.7 to 1.0 percent motional addition thereof. The as-melted material in the lybdenum, about 0.2 to 0.3 percent zirconium, about ase f Alloys A, B, C and D, was annealed one hour 0.4 to 0.6 percent hafnium, about 0.02 percent carbon, at 2300C upset forged to a P reduction about0.02 percent nitrogen, the total carbon and nitroat in e 0858 of All y he aS-melted ingot gen not exceeding 0.04 percent, and the balance tantawas o ged to percent reduction at 1400C. Excellum. lent forgeability characteristics were exhibited under Refere ce i di d to T bl I] hi h li h the described conditions. Generally the forging is carsile test results for the compositions set forth in Table 40 ried out in the temperature range from 12000 to l. Included in Table II are test results for two compositions Alloy F and Alloy T-222 each of which is outside the scope of the present invention.

TABLE II 1400C. The upset forged slab was annealed one hour at 1650C and then was cold rolled percent with ex- Pe ee e fittir nsekta eei lae .welda l t 9 Tensile Properties (Tested In The As-Worked Condition) Test 0.2% Offset Ultimate Percent Temper Yield Strength Tensile Elonga- Alloy ture(F) (psi) Strength (psi) tion A R.T. 160,200 168,700 3 R.T.(l) 127,100 134,000 24 2000 95,300 98,800 3.90 2400 66,200 72,700 6.60 B R.T.(l) 106,600 118,500 26 2400 61,300 67,900 6 C R.T.(l) 101,000 111,700 26 2000 77,600 80,100 5 2400 59,200 61,100 7 D R.T.(l) 108,600 125,200 25 2400 74,200 79,000 7 E R.T.(l) 85,000 105,400 25.9

2400(1) 35,400 40,900 35 F R.T.( 1) 78,600 106,800 24 2400(1) 29,400 43,200 36 T-222 R.T.(l) 107,600 111,800 25 2400(1) 37,000 52,500 25 2000(2) 89,200 100,000 10 2400(2) 56,000 61,300 24 l. Annealed one hour at 1650C (3000F) prior to testing 2. Cold worked. stress relieved one hour at 2000F.

the material was excellent in that tungsten inert gas welded butt joints and electron beam welded joints were ductile in bending at 200F over a mandrel with a radius 1.8 times the sheet thickness.

The alloys of the invention were then tested to determine their creep properties at.2400F in a vacuum of 10 Torr. Alloy T-222 was similarly tested. The results are set forth in Table 11] below.

" TABLE 111 Creep Properties Annealed One Hour at 1650C (3000F) Prior to Testing Test Total Time To Temper- Stress Test Time Elonga- Elongate Alloy ature (F) (psi) (hrs.) tion A 2400 15,000 192 0.26 over 200 (est) B 2400 15,390 94 0.2 over 100 (est) C 2400 15,000 90.9 1.03 91 D 2400 15,000 102 1.32 90 E 2400 15,000 554 2.53 262 F 2400 15,000 209 2.19 115 T-222 2 400 15,000 235 4.7 83

mtsrv' d'aryphase'ssfaimetamibiae ."sigai'seam in this respect is the fact that the improved creep properties are associated only with the dimetal phases as illustrated in FIG. 2. Alloy E, which has outstanding creep properties was subjected to X-ray diffraction analysis of chemically extracted dispersed phases. The results are set forth hereinafter in Table IV.

NAS-56(8W-1Re- 0.7Hf-.025C) 1 hr. at l650Cl3000F 193 hrs. at 13l5C/2400F NAS-56 (8W-1Re- 0.7Hf-.025C) 1 hr. at 1650Cl3000F 308 hrs. at l3l5Cl2400F and 12,690 psi and 15,000 psi d Intensity Phase cl Intensity Phase 3.15 VVW Ht'O 2.69 M HCP 2.82 VVVW HfO 2.62 VVVW (2.37Ta C) 2.69 M HCP 2.47 S HCP 2.62 VVW (2.37Ta C) 2.37 VS HCP 2.47 S HCP 1.82 MS HCP 2.37 VS HCP 1.555 MS HCP 1.82 MS HCP 1.405 MS HCP 1.555 MS HCP 1.345 HCP 1.405 MS HCP 1.315 MS HCP 1.345 W HCP 1.300 M HCP 1.315 MS HCP 1.2355 W HCP 1.300 M HCP 1.183 W HCP 1.2355 W HCP 1.125 W HCP 1.183 W HCP 1.042 M HCP 1.125 W HCP 1.028 W HCP 1.042 M HCP 0.997 MSv HCP 1.028 W HCP 0.968 M HCP 0.997 MS HCP 0.941 MW HCP 0.968 M HCP 0.929 MW HCP 0.941 MW HCP 0.910 W HCP 0.929 MW HCP 0.897 W HCP 0.910 W HCP 0.867 MS HCP 0.897 W HCP 0.844 M HCP 0.867 MS HCP 0.796 M HCP 0.844 M HCP 0.7875 VW HCP 0.796 M HCP 0.796 W HCP 0.7875 VW HCP 0.776 W HCP 0.796 W HCP 0.776 W HCP Note:

HCP with a.,=3.11 A HCP with a, =3.11 A 2,, =4.95 A c, =4.95 A ([11 =1.59 c/a =1.59

(Ta, W, Re, Hf), C,

From the test results set forth hereinbefore it is clear that the stable secondary phase is that of the dimetal carbide. Since other test results show lattice constants a, within the range of 3.10 A. to 3.15 A. and c, within the range between 4.92 A. and 4.96 A. thus indicating very minor substitution of hafnium tungsten and rhenium for tantalum. This is in excellent agreement with the values of a, 3.106 A. and c 4.945 A. reported for the compound Ta C.

As stated previously, the alloying components must be limited to percent total substitutional solute components. The alloys (A, B, C, D and E) each contain less than 10 percent substitutional solute (W+ Mo+ Re+Zr+ Hf) as opposed to approximately 12.5 W+Hf present in T-222. The amount of substitutional solute elements present in the alloy is an important consideration since fabricability is strongly affected by the amount of such solid solution strengtheners present. The more total strengthener present the less fabricable the alloy tends to be and therefore, more energy and heavier equipment is required to work the alloy. The substitution of rhenium for part of the tungsten in Alloys A, B and E effects an improvement in creep resistance. The addition of rhenium apparently increases the shear modulus of the matrix and decreases the diffusivity of matrix, thereby decreasing the creep rate.

The extraordinary effect of the rhenium addition in Alloy E is demonstrated by creep testing an alloy having the composition of Alloy F. Alloy F is similar in all respects to Alloy E except that in Alloy E 1 percent rhenium has been substituted for an equal weight of tungsten.

Comparing the results set forth in Table III for Alloys E and F it is seen that the time to elongate 1 percent is doubled by the substitution of 1 percent rhenium for 1 percent tungsten. This is quite surprising in view of the fact that the low temperature-short time properties such as yield strength and ductility (elongation) are substantiallly unaffected by this same substitution. Thus the preferred alloy contains about 1 percent rhenium.

The element oxygen is undesirable in the alloys of this invention since the oxides are formed in a particle size which is too large to effectively pin dislocations. Further, the oxygen removed hafnium from solution and reduces the amount available for reaction with the carbon and nitrogen. Accordingly, oxygen in these alloys is preferably restricted to an amount of 100 parts per million or less. I

Thus, there have been described tantalum-base alloys having exceedingly useful properties and which are suitable for application at elevated temperatures in a vacuum environment.

It will be understood by those skilled in the art that although the invention'has been described in connection with preferred alloys, modifications and variations may be employed without departing from the underlying spirit and scope of the invention.

1 claim as my invention:

1. A tantalum base alloy particularly adapted for use at elevated temperatures under high vacuum consisting essentially of, by weight, from about 4 percent to about 8.5 percent tungsten, from about 0.5 percent to about 3.0 percent of at least one metal selected from the group consisting of molybdenum, rhenium and mixtures thereof, the molybdenum when present being within the range of up to about 1.25 percent andthe rhenium, when present being within the range of up to about 3 percent, up to about 1.5 percent of at least one metal selected from the group consisting of hafnium. zirconium and mixtures thereof, the hafnium, when present being within the range of up to about 1.5 percent and the zirconium when present being within the range of up to about 0.75 percent, from about 0.01 percent to about 0.04 percent of at least one element selected from the group consisting of carbon, nitrogen and mixtures thereof, the carbon, when present being within the range of traces to about 0.03 percent and the nitrogen when present being within the range traces up to about 0.03 percent and the balance tantalum with incidental impurities, the alloy being fabricable while having a DBTT below room temperature and the alloying elements being selected within the ranges stated to produce a microstructure characterized by exhibiting a dispersed secondary phase of a dimetal carbide of the (Ta, W, Mo, Re, Hf, Zr) C type, is substantially free of monometal carbides and which alloy exhibits excellent creep properties.

2. An alloy as set forth in claim 1 wherein tungsten is present in amounts of from 4 to 8 percent, molybdenum is present in amounts of from 0.5 to 1.25 percent, rhenium is present in amounts of from 0.5 to 1.25 percent, hafnium is present in amounts of from 0.1 to 1.5 percent and nitrogen is present in amounts of from 0.01 to 0.03 percentas a partial substitute for carbon.

3. An alloy as set forth in claim 1 wherein tungsten is present in amounts of from 5.5 to 5.8 percent, molybdenum is present in amounts of from 0.6 to 0.9 percent, rhenium is present in amounts of from 1.4 to 1.6 percent, hafnium is present in amounts of from 0.2 to 0.3 percent, zirconium is present in amounts of from 0.1 to 0.2 percent, carbon is present in an amount of from 0.02 percent and nitrogen is present in an amount of about 0.02 percent.

4. An alloy as set forth in claim 1 wherein about 6.3 to 7.2 percent of tungsten is present, molybdenum, when present amounts to about 0.7 to 1 percent, rhenium, when present, amounts to about 1.4 to 1.6 percent, about 0.2 to 0.3 percent zirconium is present, about 0.4 to 0.6 percent hafnium is present, about 0.02 percent nitrogen is present, and traces to about 0.02 percent carbon is present.

5. An alloy as set forth in claim 1 wherein from 6.8 to 7.2 percent tungsten is present, from 1.4 to 1.6 percent rhenium is present, about 0.2 to 0.3 percent zirconium is present and about 0.02 percent nitrogen is present as a partial substitute for carbon.

6. An alloy as set forth in claim 1 wherein from 6.8 to 7.2 percent tungsten is present, from 0.7 to 1 percent molybdenum is present, from 0.2 to 0.3 percent zirconium is present and about 0.02 percent nitrogen is present as a partial substitute for carbon.

7. An alloy as set forth in claim 1 wherein from 6.3 to 7.2 percent tungsten is present, from 0.7 to 1 percent molybdenum is present, from 0.2 to 0.3 percent zirconium is present, from 0.4 to 0.6 percent hafnium is present, about 0.02 percent carbon is present and about 0.02 percent nitrogen present. V H H 8. A highly workable tantalum-base alloy having exceptional resistance to creep deformation consisting essentially of, by weight, from about 7 to 8.5 percent tungsten, from 0.5 to 1.5 percent rhenium, from 0.5 to 1.25 percent hafnium, from 0.02 to 0.03 percent carbon and the balance essentially tantalum with small amounts of impurities, said alloying components being selected within the ranges stated to produce a microan amount oi about 8 percent, rhenium is present in an amount of about 1 percent, hafnium is present in an amount of about 0.7 percent and carbon is present in an amount of about 0.025 percent carbon. 

2. An alloy as set forth in claim 1 wherein tungsten is present in amounts of from 4 to 8 percent, molybdenum is present in amounts of from 0.5 to 1.25 percent, rhenium is present in amounts of from 0.5 to 1.25 percent, hafnium is present in amounts of from 0.1 to 1.5 percent and nitrogen is present in amounts of from 0.01 to 0.03 percent as a partial substitute for carbon.
 3. An alloy as set forth in claim 1 wherein tungsten is present in amounts of from 5.5 to 5.8 percent, molybdenum is present in amounts of from 0.6 to 0.9 percent, rhenium is present in amounts of from 1.4 to 1.6 percent, hafnium is present in amounts of from 0.2 to 0.3 percent, zirconium is present in amounts of from 0.1 to 0.2 percent, carbon is present in an amount of from 0.02 percent and nitrogen is present in an amount of about 0.02 percent.
 4. An alloy as set forth in claim 1 wherein about 6.3 to 7.2 percent of tungsten is present, molybdenum, when present amounts to about 0.7 to 1 percent, rhenium, when present, amounts to about 1.4 to 1.6 percent, about 0.2 to 0.3 percent zirconium is present, about 0.4 to 0.6 percent hafnium is present, about 0.02 percent nitrogen is present, and traces to about 0.02 percent carbon is present.
 5. An alloy as set forth in claim 1 wherein from 6.8 to 7.2 percent tungsten is present, from 1.4 to 1.6 percent rhenium is present, about 0.2 to 0.3 percent zirconium is present and about 0.02 percent nitrogen is present as a partial substitute for carbon.
 6. An alloy as set forth in claim 1 wherein from 6.8 to 7.2 percent tungsten is present, from 0.7 to 1 percent molybdenum is present, from 0.2 to 0.3 percent zirconium is present and about 0.02 percent nitrogen is present as a partial substitute for carbon.
 7. An alloy as set forth in claim 1 wherein from 6.3 to 7.2 percent tungsten is present, from 0.7 to 1 percent molybdenum is present, from 0.2 to 0.3 percent zirconium is present, from 0.4 to 0.6 percent hafnium is present, about 0.02 percent carbon is present and about 0.02 percent nitrogen is present.
 8. A highly workable tantalum-base alloy having exceptional resistance to creep deformation consisting essentially of, by weight, fRom about 7 to 8.5 percent tungsten, from 0.5 to 1.5 percent rhenium, from 0.5 to 1.25 percent hafnium, from 0.02 to 0.03 percent carbon and the balance essentially tantalum with small amounts of impurities, said alloying components being selected within the ranges stated to produce a microstructure exhibiting a dispersed secondary phase of a dimetal carbide of the (Ta, W, Mo, Re, Hf, Zr)2 C1 x type and which is substantially free of mono-metal carbides.
 9. The alloy of claim 8 wherein tungsten is present in an amount of about 8 percent, rhenium is present in an amount of about 1 percent, hafnium is present in an amount of about 0.7 percent and carbon is present in an amount of about 0.025 percent carbon. 